Found it in the Arch wiki where it's called "Poor man's tiling window manager". It uses wmctrl to rearrange windows. This is an old python2 script and fossapup only has python3. Knowing next to nothing about coding I've used a tool to convert it. Still needed to read some answers on stackoverflow to make it run. I've changed it a bit at the same time. Here are the changes:
converted to python3 (by 2to3 and some tweaking)
applied the proposed fix for the maximized windows bug (from github)
replaced xdotool with xwininfo as the latter is already installed
added a config option to ignore minimized windows
added a config option for the default layout that is used by swap and cycle issued before simple, vertical or horizontal commands
swap and cycle now works with vertical and horizontal layouts too
Tested on Fossapup64-9.5 only.
Usage:
download and gunzip to /root/my-applications/bin (make executable)
create JWM keyboard shortcuts for action exec:stiler.py command where command can be: left right simple vertical horizontal swap cycle maximize max_all (one command at a time)
Notes:
left and right are supported by JWM itself (maxleft and other actions)
Just found this fork Requires python2 too, uses xprop and has more functions.